Make the check intervall configurable

[cliffparnitzky]

Currently the hourly Cron checks every hour (when triggered by real Cron).

In some cases it would be nice to configure the Monitoring System to check in smaller intevalls (e.g. every 15 minutes). A global value and overwriting for each system would be nice.

[cliffparnitzky]

This will have an impact on all other modules, that assume teh 1 hour check (e.g. timeline, compression).

[cliffparnitzky]

Timeline remembers the timestamp to calculate the difference between two tests ... so, there will not be any problemes.